This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We focus on various cytidine deaminases, including the antiviral protein APOBEC3G and adenosine deaminases acting on tRNA. All canonical transfer RNAs (tRNAs) have a uridine at position 8, involved in maintaining tRNA tertiary structure. However, the hyperthermophilic archaeon Methanopyrus kandleri harbors 30 (out of 34) tRNA genes with cytidine at position 8. We demonstrate C-to-U editing at this location in the tRNA's tertiary core, and present the crystal structure of a tRNA-specific cytidine deaminase, CDAT8, which has the cytidine deaminase domain linked to a tRNA-binding THUMP domain. CDAT8 is specific for C deamination at position 8, requires only the acceptor stem hairpin for activity, and belongs to a unique family within the "cytidine deaminase[unreadable]like" superfamily. The presence of this C-to-U editing enzyme facilitates the proper folding and functionality of all M. kandleri tRNAs. We also work on the mechanism of amyloid detection by Thioflavin T.